Thursday, 22 March 2012

LEDs in headlamps offer high differentiation potential for automakers

  • Light color even closer to daylight
  • Strong increase in LED signal functions in headlamps
  • In future, it will be possible to implement intelligent lighting functions without mechanisms

Hella Adaptive Full LED headlamp prototype in Opel Signum
Article of 22.03.2007
March 2007. These days, LEDs (Light Emitting Diodes) are used as light sources at the vehicle rear and, increasingly, also at the vehicle front, for signal functions. In the coming years, this trend will grow considerably. The first full LED headlamps, which will generate low beam and high beam, will be on the market in 2007 and 2008. They open up new, far reaching styling options and therefore great differentiation potential for automakers. At Hella, the next LED headlamp generation is already being developed: from about 2012, freely addressable LED arrays (assemblies) will make new, active lighting functions possible.
Extremely long service life (up to 10,000 hours), a high level of reliability, low package space, rapid switch-on behavior and, depending on the function, up to 80 percent less power consumption: these advantages have now secured LEDs a firm position on the vehicle rear. Consistently, these advantages are being used especially in the case of central high mounted stop lamps, but also more and more in combination rear lamps. For at least six years, LEDs have been offered which generate white light. The first applications in the front-end area are the position light and daytime running light. The rapid development of their performance now makes them usable for low beam and high beam as well.
In relation to this, the light color of the white LEDs (color temperature approx. 5,500 kelvin) approaches that of daylight (approx. 6,000 kelvin) considerably more than xenon light (approx. 4,000 kelvin). This fits in with what humans are used to seeing. Car drivers do not tire as quickly and are more relaxed when driving. In addition, the white LEDs are also a convincing choice thanks to their fast response time, and it is possible to cover two functions (such as position light and daytime running light) using a single light source. Their luminous efficiency is increasing rapidly: while today it is over 40 lm/W, there are already prototypes in research institutes with approximately 130 lm/W (in comparison with xenon light: approx. 90 lm/W, halogen light 20 lm/W).

LED headlamps have a modular design
As high luminous power is required for low beam and high beam, Hella chooses multi-chip LEDs for the LED headlamp. Special chip arrangements and housing geometries make it possible to generate the cut off line without additional mechanical shading. This allows compact headlamp systems to be realized. The light distribution is designed in an almost modular manner:
  • A symmetrical basic light distribution with large dispersion provides homogeneous illumination of the close range in front of the vehicle.
  • In the central area close to the cut off line, a field with high illuminance overlaps this basic light distribution and generates the range of the low beam on the left hand side of the road (oncoming traffic).
  • The long range asymmetrical "light finger" on the right hand side of the road provides the necessary long range orientation for the driver.
  • The addition of further optical modules makes it possible to realize AFS lighting functions, such as town, country, motorway and adverse weather light. Triggering takes place by means of electronic control units which then switch on or switch off further light sources according to the current driving situation.

The different light distributions are generated by means of different optical modules that are matched to each other. Variation possibilities with regard to the arrangement and number of the modules open up new, far reaching styling options and therefore great potential for differentiation for automakers.

LED low beam is close to the Xenon values
Thus, the full LED headlamp prototype from Hella has been realized in an Opel Signum using different transmissive optical elements. Six optical units generate the low beam. Three further units are added for the high beam, a special module is sufficient for the cornering light. Daytime running light/position light and direction indicator with LEDs complete the full LED headlamp, which, in the case of low beam, is already very close to the Xenon values. With reference to the number of LEDs and their performance, these and other prototypes are designed in such a way that luminous power similar to a Xenon headlamp can be achieved in 2008. With regard to this, the Hella lighting specialists take into consideration the continuously increasing performance of white LEDs which is to be expected by then. 

Hella Adaptive Full LED headlamp in Opel Signum

LED - Array
Hella is working with LED arrays (assemblies) as light sources for future headlamps. They consist of several individually addressable LED chips which are arranged in accordance with a predefined pattern onto a carrier substrate. The individual chips are triggered by means of pulse width modulation. This allows, on the one hand, the switching on and switching off of individual LED chips and therefore the generation of different cut off line geometries, on the other hand, it also allows an intensity modulation in the light distribution. In addition to the implementation of AFS lighting functions without mechanisms, LED arrays make possible, in conjunction with forward looking vehicle sensor technology and intelligent triggering electronics, the realization of active lighting functions such as marker light or glare free high beam. 

Approval situation for LEDs in headlamps:
In the USA, headlamps with LEDs for the main lighting functions are already permitted according to the SAE standards valid there. In Europe, or in the scope of application of the ECE regulations, corresponding approval can be expected by 2008. Signal functions in the headlamp (direction indicator, side marker light, position light and daytime running light, as well as cornering light) with LEDs are today permitted both in the ECE and SAE scope of application.

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